Since the advent of inflatable devices and other types of apparatus employing compressed air or other "fluid" medium (hydraulics, etc.), the regulation of fluid pressure within such devices and apparatus has been an important consideration. Many of the early inventions relating to inflatable devices pertained to means for inflating and deflating the devices, such as compressed air supply sources, air pressure regulators, manual, solenoid and pilot operated directional control valves, etc. In addition, various means were developed for measuring the air pressure within such devices, such as pneumatically actuable gauges.
More recent inventions relating to inflatable devices have often combined the functions of measurement and inflation. For example, the Dudar U.S. Pat. No. 4,212,334 issued July 15, 1980 describes a unit having a series connected air supply, solenoid-actuated valve and spring loaded diaphragm to allow inflation of a vehicle tire only if the tire first has a selected minimum air pressure.
Still further, other inventions are directed to more sophisticated regulation of air pressure within inflatable devices and other pneumatic apparatus. That is, these inventions are directed to the regulation of actual air pressure within pneumatic apparatus in accordance with a desired air pressure which, in turn, may be adjustable. It is obvious that one may manually measure the air pressure within a pneumatic apparatus and then activate well-known valve and air supply devices to either add or exhaust air from the pneumatic apparatus. However, accuracy in achieving an exact desired air pressure in this manner can be difficult. In addition, there are instances where automated or remote control of such functions is substantially advantageous.
For example, various types of vehicles having pneumatic tires are adapted for use on differing terrains Such "cross country" vehicles must be capable of operation on both hard surfaces (e.g. paved roads) and soft terrain (e.g. sand dunes, mud, swamp land, etc.).
At high speeds on paved surfaces, it is desirable to have a relatively high tire pressure and small cross sectional area of contact between the tires and the surface. Conversely, on terrain such as sand and mud or swamp land, it is desirable to reduce tire pressure and increase tire/surface contact area. However, it is sometimes difficult and time consuming, if not impossible, for the vehicle operator to leave the vehicle cab and manually inflate or deflate tires. Also, it is difficult to achieve air pressure accuracy by manual operations.
Systems for remotely regulating air pressure in pneumatic tires on cross country and amphibious vehicles are described in the Williams U.S. Pat. No. 2,685,906 issued Aug. 10, 1954 and Brockmann U.S. Pat. No. 4,313,483 issued Feb. 2, 1982. The system described in the Williams patent employs a source of compressed air and a series of pneumatically controlled valves to inflate or deflate a selected number of vehicle tires in accordance with operation of a manually actuable position switch in the vehicle cab. A pressure gauge mounted to the system measures actual tire air pressure. When the actual air pressure attains a desired value, the operator turns off the position switch, thereby causing the valves to cease inflation or deflation of the tires.
The Brockmann system includes a conventional compressed air source connected to a magnetic valve controlled by a signalling relay key and a pressure relief key. A line switch is provided to select a desired air pressure and actuation of the signalling key will cause the magnetic valve to open and control the flow of compressed air to vehicle tires.
The compressed air is supplied through a ring chamber to a parallel connection of a wheel control valve and a relief valve. The wheel control valve is, in turn, connected to the tire. The wheel control and relief valves include spring loaded inner valve heads pnuematically interconnected in a manner so that attainment of maximum tire pressure causes the relief valve to rapidly force the wheel control inner valve head closed. The relief valve also includes a back flow valve head to assist in venting interconnecting air lines.
Another type of vehicle mounted pneumatic control system for regulating the air pressure in pneumatic vehicle tires and allowing the vehicle operator to remotely adjust the tire air pressure is depicted in FIG. 1 as pneumatic tire pressure control system 100. The system 100 is a pneumatically operated system adapted to monitor and adjust the air pressure in vehicle tires so as to maintain a desired air pressure. System 100 may be advantageously employed in cross country vehicles so as to allow the vehicle operator to remotely modify the tire air pressure for purposes of optimizing maneuverability on differing types of terrain.
Referring to FIG. 1, the system 100 is vehicle mounted (the vehicle not being shown) and includes a vehicle air supply source 102. The supply source 102, which can be in the form of an air compressor or other conventional means, supplies air through line 104 to a compressed air receiver 106 The receiver 106 can be in the form of a storage tank or similar facility, and may have multiple uses, such as supplying compressed air to a vehicle brake system (not shown) over a brake air supply line 108.
The compressed air supply 106 is connected to an output main line 107 having a low pressure protection valve 110 in series therewith. The protection valve 110 is well-known in the art of pneumatic system design and provides a means for assuring adequate pressure for the vechicle brake system in the event of an air loss in system 100 beyond the protection of valve 110, where such air loss causes a low pressure condition in the receiver tank 106.
The compressed air output main line 107 is connected to a regulator input main line 112. Main line 112 is connected through a check valve 114 to a parallel connection of separate regulator input lines 116, 117, 118 and 119. The regulator input lines 116-119 are connected as primary input lines to manifold mounted air pressure regulators 122, 123, 124 and 125, respectively. The regulators 122-125 are conventional in design and provide secondary outputs at predetermined desired air pressures. The regulators 122-125 can, for example, provide secondary outputs at 75, 30, 18 and 10 PSI, respectively.
The outputs of regulators 122 and 123 are connected as input lines to a dual inlet, three-position, detented, five-way hand operated directional control valve 128. Correspondingly, the regulators 124 and 125 have their outputs connected as input lines to a dual inlet, two-position, detented, five-way hand operated directional control valve 130. The directional control valves 128 and 130 are also conventional in design and provide a means for manually selecting a "desired" air pressure corresponding to the output air pressure of one of the regulators 122-125.
Output lines of the directional control valves 128 and 130 are connected as inputs to a triple shuttle valve assembly 132. The shuttle valve assembly 132 has an output air supply line 134 which, during operation of the control system 100, will have a pressure corresponding to the desired air pressure for vehicle tires 136. Air supply line 134 is connected as an input line to a modulator valve 148 comprising a dual inlet, three position, five-way, spring centered, pilot operated directional control valve, the operation of which is subsequently described herein.
The pressure in the vehicle tires 136 is monitored in an air line 138 having a pressure hereinafter referred to as the "actual" tire pressure. Line 138 is connected through a damping orifice 140 and static tank 142 to an air gauge line 144 which, in turn, is connected to a vehicle dashboard mounted air gauge 146. The damping orifice 140 and static tank 142 serve a dual purpose. Namely, the static tank 142 effectively provides a sampling point for determining the actual tire pressure. Secondly, the combination of the damping orifice 140 and static tank 142 provide a means to minimize the effects of pressure transients in the tires 136 due to momentary changes in road and terrain conditions. Line 138 is also directly connected to an air line 139 which, in turn, is connected as an input line to the modulator control valve 148.
Referring again to the compressed air main supply line 107, it is connected to an air supply line 152 which provides a compressed air supply to the inlets of modulator control valve 148. The port supplying air to the inflation port of valve 148 receives its supply directly from line 152. The port supplying air to the deflation port of valve 148 receives its supply through a low pressure protection valve 150 The low pressure protection valve 150, along with bleed orifice 174, protects against unwanted tire deflation. The modulator valve 148 is conventional in design and employs spring loaded diaphragm chambers which effectively compare the desired air pressure in line 134 with the actual air pressure in line 139. Output lines from the valve 148 include an inflate control line 160 and a deflate control line 162. In accordance with the comparison of air pressures on lines 134 and 139, air is supplied to one or neither of the control lines 160 and 162. Lines 160 and 162 are connected to separate spring loaded diaphragm chambers of a three position, three way, spring centered, pilot operated directional control slave valve 170. The slave valve 170 also includes a direct air supply connection through line 149 from the compressed air main supply line 107. An output line 172 directly connects the slave valve 170 to air valves (not shown) on the tires 136.
In operation of pressure control system 100, the vehicle operator will manually manipulate the directional control valves 128 and 130 so as to select a desired air pressure For example, should the vehicle operator select a desired air pressure of 30 PSI, the directional control valve 128 will be positioned so as to allow compressed air from the output of regulator 123 to be applied through the shuttle valve assembly 132 onto air line 134. As previously described, line 134, having an air pressure corresponding to the desired air pressure, applies the air pressure to the associated spring loaded diaphragm chamber of the modulator valve 148. Correspondingly, the actual air pressure of the tires 136 is applied through line 139 to a separate spring loaded diaphragm chamber of valve 148. Valve 148 operates to compare the desired air pressure on line 134 with the actual air pressure on line 139. Should the desired air pressure on line 134 exceed the actual air pressure, the control valve 148 will operate to effectively connect air supply line 152 to the inflate control line 160.
The compressed air flowing in line 160 will cause the slave valve 170 to operate in a manner so as to provide a direct connection of the compressed air supply line 149 to the main tire supply line 172, thereby causing the tires 136 to be inflated. The slave valve 170 remains in an inflation state until the actual air pressure is increased to the desired air pressure level, at which time the modulator valve 148, sensing the air pressure correspondence, will shut off air supply to control line 160.
If the desired air pressure in line 134 were less than the actual air pressure in line 139, the modulator valve 148 would effectively connect the compressed air supply line 152 through the low pressure protection valve 150 to the deflate control output line 162, provided there is a sufficiently high level of pressure in air supply line 152 to pilot the protection valve 150 into an open state. The compressed air in line 162 would thereby cause the slave valve 170 to be connected to main tire supply line 172 in a manner such that air would be exhausted from the tires 136. The slave valve 170 would remain in an exhuast state until such time as the actual air pressure in line 139 were reduced to a value corresponding to the desired air pressure in line 134.
In the system 100 depicted in FIG. 1, the portion shown in dotted lines with numerical reference 154 can be characterized as a tire pressure control assembly which comprises means for manually selecting a desired air pressure and generating a pneumatic signal having an air pressure corresponding to the desired air pressure. The control assembly 154 also comprises means for comparing the desired air pressure signal with a pneumatic signal having a pressure corresponding to the actual air pressure. In addition, the control assembly 154 is responsive to the resultant comparison to control a slave valve 170 so as to selectively inflate or deflate the tires 136 to achieve the desired air pressure.
Although a pneumatic control system corresponding to the previously described control system 100 represents a substantial advance over previously known systems, various types of problems remain with such a system. Although the system 100 has substantial durability and reliability, it would clearly be advantageous to improve the same. Control systems employing mechanical assemblies, pneumatic and other fluid devices are known to be readily susceptible to contamination, wear and resultant breakdown. It is also advantageous to achieve a substantial accuracy of pressure control, particularly at low pressures. In a physically realized system corresponding to system 100, a fairly wide deadband will exist symmetrical about a desired air pressure at which no correction in actual air pressure will occur. The deadband occurring in the system can be influenced by a number of factors such as valve spool clearances, O-ring and seal friction, contamination in the system and operating temperatures. In general, control systems employing pneumatic assemblies, including such elements as flexible diaphragms, can develop significant problems at low operating temperatures due to stiffness of materials and other undesirable phenomena which are known to occur at lower than normal temperatures. These phenomena can be a particular disadvantage in vehicles such as the previously referenced cross country vehicles which must reliably operate in severe environments. Finally, the difficulty of installation on vehicles can also be an important consideration, especially since tire pressure control systems are not commonly be installed on vehicles during factory manufacture.
In control systems for regulating various types of physical parameters, it is known to employ electrical circuitry for purposes of providing signals to control nonelectrical functions. It is also known to employ transducer devices to generate electrical signals having magnitudes representative of the values of nonelectrical physical parameters. For example, in the Knubley U.S. Pat. No. 4,333,491 issued June 8, 1982, an electropneumatic apparatus is described for accurately inflating tires to a predetermined air pressure. The system includes a pressure transducer for measuring the air pressure of a line directly connected to a pneumatic tire of a vehicle and for generating an output voltage proportional to the measured pressure. The resultant voltage is applied to a control unit having manual controls for setting the desired air pressure.
Specifically, the transducer voltage is applied as an input to a voltage/frequency converter circuit which generates a variable frequency output signal. The variable frequency output signal is applied, during one second of each five second interval, to a four stage binary coded decimal counter which provides a count of the number of excursions of the variable frequency signal during the one second interval. Signals are then applied through various digital counting circuits to operate relays which control air being supplied to the tire. In summary, the system operates in five second cycles with separate one second intervals. Air is applied during a first interval and air presure is measured and compared to desired air pressure during a second interval.
The Carman et al U.S. Pat. No. 3,726,307, issued Apr. 10, 1973, also describes the use of a pressure to voltage transducer circuit. The circuit is employed in a system to provide a pilot pressure to a regulator in accordance with a desired pilot pressure so as to regulate fluid from a supply source to a regulated supply storage device.
ln the Carman et al system, a voltage representative of the actual pilot pressure is applied through various circuitry to inputs of low and high level comparator circuits. Signals representative of a deadband around a desired pilot pressure are also applied to the comparators. Output signals of the comparators are utilized to drive solenoid operated valves connected to an accumulator diaphragm at an inlet to the fluid regulator. The control circuitry described in the Carman et al patent does not appear to provide for lowering of fluid pressure in the supply storage device.
Although circuitry is known as heretofore described for controlling nonelectrical parameters, the implementation of such circuitry can include problems in addition to those previously described with respect to pneumatic and mechanical control system apparatus for use in apparatus such as tire inflation/deflation assemblies. For example, the circuitry should not exhibit substantial power dissipation requiring heat sinks or other extraordinary means of dissipation. Accordingly, and for purposes of achieving a relatively compact and lightweight design, the designer must attempt to minimize "hardware count". However, the system must also provide sufficient power to operate switching devices capable of switching electromechanical and electropneumatic assemblies. Circuit design considerations for effecting these features are often somewhat opposed to each other.
An important consideration in the design of any control system utilizing electrical signals having polarities and magnitudes representative of actual values of nonelectrical parameters, i.e. analog circuitry, is stability and retention of accuracy, even in the event of phenomena such as power source drift. Similarly, it is highly advantageous to achieve a system control which retains accuracy and does not respond in the event of electrical and nonelectrical transient conditions.
Another problem associated with system control circuits, especially those employing feedback concepts, is the well-known problem of "overshoot" and hunting which can cause a circuit to oscillate around desired values. Furthermore, the means employed for minimizing hunting and overshoot should not add a substantial amount of circuitry to the system. Finally, the system control should provide for a relatively small operational deadband and should provide ease of installation, including the capability of retrofit to existing control systems.